Fuel cells directly convert chemical energy of a fuel gas (i.e., hydrogen (H2)) into electrical energy.
Fuel cells are capable of generating direct current. Fuel cells continuously generate electricity using fuel and air supplied from the outside, in contrast with conventional electric cells.
In particular, fuel cells are an electric generator which directly converts chemical energy of a fuel gas into electrical energy via electrochemical reaction using hydrogen (H2) contained in a hydrocarbon compound, such as methanol or natural gas, and oxygen (O2) in air, as fuel gases.
Fuel cells are a clean energy converter with high efficiency that use both electricity, generated by electrochemical reaction of a fuel gas with an oxidizing gas without any combustion, and heat as a by-product thereof.
Based on the electrolyte used, fuel cells are divided into phosphoric acid fuel cells operating at a temperature of about 150 to 200° C., polymer electrolyte fuel cells and alkaline fuel cells operating at a temperature range of about room temperature to 100° C. or less, molten carbonate fuel cells operating at a high temperature of about 600 to 700° C., and solid oxide fuel cells operating at a high temperature of about 1,000° C. or more.
These fuel cells have similar operation mechanisms, but they are different from each other in terms of fuel type, operation temperature, and catalyst and electrolyte used.
FIG. 1 is a sectional view illustrating an electricity generation mechanism in a unit cell of a fuel cell.
As shown in FIG. 1, a unit cell of the fuel cell includes of a drying layer 16 made from a Nafion solution, Nafion films 15, 15′ arranged on the opposite sides of the drying layer 16, platinum/carbon catalyst layers 14, 14′ acting as electrodes, teflon-treated carbon cloths 13, 13′, bipolar plates 12, 12′ and metallic endplates 11, 11′, which are laminated in this order.
FIG. 2 is a plan view illustrating the bipolar plates 12, 12′ in FIG. 1.
The electricity generation mechanism of the fuel cell will be described with reference to FIGS. 1 and 2.
Hydrogen (H2) gas, acting as a fuel gas, which is supplied from a gas flow channel C in one bipolar plate 12, reacts with a platinum/carbon catalyst of a positive electrode 14 and releases electrons to form hydrogen ions.
The hydrogen ions pass through polymer electrolytic films 15, 15′ and the Nafion drying layer 16 to the opposite negative electrode 14′.
Oxygen (O2) gas supplied from a gas flow channel C′ in another bipolar plate 12′ is reduced by the electrons, which are introduced into the negative electrode 14′ via an external circuit, thereby forming an oxygen ion. The oxygen ion (O2−) reacts with the hydrogen ions (H+) in the negative electrode 14′ to generate water (H2O) on the surface of the negative electrode 14′.
This water is discharged together with remaining oxygen gas into an exit of the gas flow channel C′. At this time, electrons generated by the catalyst reaction move through the external circuit to generate electricity.
The performance of the bipolar plates 12, 12′ serving as the gas flow channels C, C′ has a great influence on the generation system of fuel cells. The bipolar plates 12, 12′ must have superiority in various characteristics, such as electrical conductivity, mechanical strength, corrosion resistance and thermal stability.
In conventional cases, metallic bipolar plates, carbon bipolar plates, and carbon composite bipolar plates were commonly used as the bipolar plates.
The metallic bipolar plates have the disadvantage of poor corrosion resistance. Disadvantages of the carbon bipolar plates are high production costs and low mechanical strength. The carbon composite bipolar plates have a problem with low electrical conductivity.
Thermoplastic resin-based bipolar plates were developed in an attempt to solve the problems associated with conventional bipolar plates. Thermoplastic resin-based bipolar plates are produced by filling a thermoplastic resin matrix with a conductive filler to obtain electrical conductivity.
Thermoplastic resin-based bipolar plates use a polymeric thermoplastic resin as a matrix, and thus can be mass produced using injection molding techniques.
Thermoplastic resin-based bipolar plates continue to be the subject of active research and development efforts as an alternative to conventional bipolar plates because they have high corrosion resistance and mechanical strength, due to the inherent characteristics of the polymers used.
A thermoplastic resin-based bipolar plate can be produced by impregnating a conductive filler into a molten thermoplastic resin. The thermoplastic resin has inherently high viscosity, thus making it impossible to obtain a desired degree of impregnation of a conductive filler into the resin (Specifically, a theoretical value of maximum impregnation degree is 67% by volume, but an experimental value thereof is not more than 40% by volume).
When a bipolar plate is produced using a thermoplastic resin having a low degree of impregnation by a conductive filler, the resultant bipolar plate has low conductivity, thus limiting its commercial feasibility. Typically, for a bipolar plate to be commercially feasible, the bipolar plate should have an electrical conductivity of 50 S/cm or more.
In addition, a bipolar plate prepared with a low degree of impregnation can have a low flexural strength due to the inherent flowability of thermoplastic resins, thus making it difficult to obtain a desired mechanical strength.